White light emitting LEDs (“white LEDs”) are known and are a relatively recent innovation. It was not until LEDs emitting in the blue/ultraviolet part of the electromagnetic spectrum were developed that it became practical to develop white light sources based on LEDs. As taught, for example in U.S. Pat. No. 5,998,925, white LEDs include one or more one or more photoluminescent materials (e.g., phosphor materials), which absorb a portion of the radiation emitted by the LED and re-emit light of a different color (wavelength). Typically, the LED chip or die generates blue light and the phosphor(s) absorbs a percentage of the blue light and re-emits yellow light or a combination of green and red light, green and yellow light, green and orange or yellow and red light. The portion of the blue light generated by the LED that is not absorbed by the phosphor material combined with the light emitted by the phosphor provides light which appears to the eye as being nearly white in color. Alternatively, the LED chip or die may generate ultraviolet (UV) light, in which phosphor(s) to absorb the UV light to re-emit a combination of different colors of photoluminescent light that appear white to the human eye.
Due to their long operating life expectancy (>50,000 hours) and high luminous efficacy (70 lumens per watt and higher) high brightness white LEDs are increasingly being used to replace conventional fluorescent, compact fluorescent and incandescent light sources.
Typically the phosphor material is mixed with light transmissive materials, such as silicone or epoxy material, and the mixture applied to the light emitting surface of the LED die. It is also known to provide the phosphor material as a layer on, or incorporate the phosphor material within, an optical component, a phosphor wavelength conversion component, that is located remotely to the LED die (“remote phosphor” LED devices).
One issue with remote phosphor devices is the non-white color appearance of the device in its OFF state. During the ON state of the LED device, the LED chip or die generates blue light and the phosphor(s) absorbs a percentage of the blue light and re-emits yellow light or a combination of green and red light, green and yellow light, green and orange, or yellow and red light. The portion of the blue light generated by the LED that is not absorbed by the phosphor combined with the light emitted by the phosphor provides light which appears to the human eye as being nearly white in color. However, for a remote phosphor device in its OFF state, the absence of the blue light that would otherwise be produced by the LED in the ON state causes the device to have a yellowish, yellow-orange, or orange-color appearance. A potential consumer or purchaser of such devices that is seeking a white-appearing light may be quite confused by the yellowish, yellow-orange, or orange-color appearance of such devices in the marketplace, since the device on a store shelf is in its OFF state. This may be off-putting or undesirable to the potential purchasers and hence cause loss of sales to target customers.
Another problem with remote phosphor devices can be the variation in color of emitted light with emission angle. In particular, such devices are subject to perceptible non-uniformity in color when viewed from different angles. Such visually distinctive color differences are unacceptable for many commercial uses, particularly for the high-end lighting that often employ LED lighting devices.
Yet another problem with using phosphor materials is that they are relatively costly, and hence correspond to a significant portion of the costs for producing phosphor-based LED devices. For a non-remote phosphor device, the phosphor material in a LED light is typically mixed with a light transmissive material such as a silicone or epoxy material and the mixture directly applied to the light emitting surface of the LED die. This results in a relatively small layer of phosphor materials placed directly on the LED die, that is nevertheless still costly to produce in part because of the significant costs of the phosphor materials. A remote phosphor device typically uses a much larger layer of phosphor materials as compared to the non-remote phosphor device. Because of its larger size, a much greater amount of phosphor is normally required to manufacture such remote phosphor LED devices. As a result, the costs are correspondingly greater as well to provide the increased amount of phosphor materials needed for such remote phosphor LED devices.
Therefore, there is a need for improved approaches to implement LED lighting apparatuses that maintains the desired color properties of the devices, but without requiring the large quantities of photoluminescent materials (e.g. phosphor materials) that are required in the prior approaches. In addition, there is a need for an improved approach to implement LED lighting apparatuses which addresses perceptible variations in color of emitted light with emission angle, and which also addresses the non-white color appearance of the LED lighting apparatuses while in an OFF state.